Each year approximately 400,000 rotator cuff surgeries are performed in the United States. While operative techniques have improved, the re-rupture rate following rotator cuff repair remains high (30-35%). A recognized risk factor for tendon re-rupture is rotator cuff muscle degenerative atrophy. We suggest that a readily available off the shelf biomaterial that provides the appropriate regenerative cues by mimicking the properties of native skeletal muscle extracellular matrix could be used to regenerate damaged muscle tissue. To create a tissue mimetic material we have developed an innovative approach to capture the ECM secreted by populations of cells during growth in-vitro. This approach allows us to engineer materials that, like native tissues, are comprised entirely of cell derived constituents. The hypothesis of this research proposal is that we can use this ECM collection approach to build an engineered muscle mimetic (eMM) scaffold capable of regenerating damaged skeletal muscle. The following specific aims describe a development process in which the design of eMM scaffolds will be guided by the properties of native muscle tissue. Namely, skeletal muscle samples will be prepared and thoroughly characterized to serve as a design template (Aim 1). We will utilize our ECM collection approach to capture and organize the ECM secreted by muscle cells into an eMM scaffold with muscle mimetic composition and structure (Aim 2). We will test the regenerative performance of ECM scaffolds using a muscle defect model (Aim 3).